Why am I getting this strange error when working with findDependantRowset and Zend Framework?

PHP is throwing this error:

Fatal error: Call to a member function getAdapter() on a non-object in C:\Program Files\Zend\ZendServer\share\ZendFramework\library\Zend\Db\Table\Row\Abstract.php on line 869

... when I call this code:

$item = $itemMapper->find(1); // Find item with id of 1     
$item->findDependentRowset("Application_Model_DbTable_SubItem");

I have the following other classes:

models/DBTable/Item.php:

class Application_Model_DbTable_Item extend开发者_高级运维s Zend_Db_Table_Abstract {
    /** Table name */
    protected $_name = 'items';
}

models/DBTable/SubItem.php:

class Application_Model_DbTable_SubItem extends Zend_Db_Table_Abstract {

    /** Table name */
    protected $_name = 'subitems';
    protected $_referenceMap = array(
        'Item' => array(
            'columns' => 'items_id',
            'refTableClass' => 'Application_Model_DbTable_Item',
            'refColumns' => 'items_id'
        )
    );

}

All my models, mappers, and database tables are set up correctly, as far as I know. Accessing a model through the wrapper and fetchAll works fine.

My models extend Zend_Db_Table_Row. Why is it giving me this error?

FYI:

The line the error message cites looks like this:

public function findDependentRowset($dependentTable, $ruleKey = null, Zend_Db_Table_Select $select = null){

    $db = $this->_getTable()->getAdapter(); // <<<< there it is
...

I figured it out. Apparently, I was under the assumption that the result of find() returned by my mapper class was a Zend_Db_Table_Row object. I even tried using instanceOf to verify this, but I either used it incorrectly, or it lied to me.

The object returned by my mapper was simply a Model.

Calling find() on the Application_Model_DbTable_Table object returns a rowset, and current() works currently. After using this approach, I was able to call findDependentRowset() successfully.